Wireless flow sensor

ABSTRACT

Devices and methods useful for non-invasively measuring and indicating a rate of fluid flow are disclosed. In one embodiment, a sensor housing adapted to received fluid flow therethrough is provided. A radio frequency tag and a masking element can be disposed in the sensor housing. The masking element and the radio frequency tag can be configured to move relative to one another. The relative positions or movement can alter the response of the radio frequency tag to a wireless signal (which can be emitted from an external reading device, for example) and thereby indicate a rate of fluid flowing through the housing. For example, in some embodiments, the masking element can selectively cover at least part of the radio frequency tag in correspondence to the flow rate, which can change a characteristic of the radio frequency tag&#39;s response to the wireless signal.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.11/931,127, filed on Oct. 31, 2007, and entitled “Wireless Flow Sensor”,which is incorporated herein by reference in its entirety.

FIELD

The present application generally relates to devices and methods fornon-invasively monitoring and/or measuring the flow of a fluid, and moreparticularly for monitoring and/or measuring the flow of a fluid in animplantable medical device.

BACKGROUND

It is often desirable to non-invasively monitor or measure the flow of afluid, for example in an implanted medical device or in a body, and tobe able to communicate or indicate the monitoring or measurementinformation remotely.

By way of illustration, treatment of hydrocephalus can involvemonitoring the flow rate of cerebrospinal fluid through a hydrocephalusshunt. Hydrocephalus is a neurological condition that is caused by theabnormal accumulation of cerebrospinal fluid (CSF) within theventricles, or cavities, of the brain. CSF is a clear, colorless fluidthat is primarily produced by the choroid plexus and surrounds the brainand spinal cord, aiding in their protection. Hydrocephalus can arisewhen the normal drainage of CSF in the brain is blocked in some way,which creates an imbalance between the amount of CSF produced by thechoroid plexus and the rate at which CSF is absorbed into thebloodstream, thereby increasing pressure on the brain.

Hydrocephalus is most often treated by surgically implanting a shuntsystem in a patient. The shunt system diverts the flow of CSF from theventricle to another area of the body where the CSF can be absorbed aspart of the circulatory system. Shunt systems come in a variety ofmodels and typically share similar functional components. Thesecomponents include a ventricular catheter, which is introduced through aburr hole in the skull and implanted in the patient's ventricle, adrainage catheter that carries the CSF to its ultimate drainage site,and optionally a flow-control mechanism, e.g., shunt valve, thatregulates the one-way flow of CSF from the ventricle to the drainagesite to maintain normal pressure within the ventricles. It is this flowof CSF which may need to be measured.

In some cases, measuring the flow of CSF can be accomplished by a flowsensor using temperature differentials between two points, e.g., with amechanism for heating or cooling the CSF in a particular section of thecatheter. However, it would be advantageous to provide a flow sensorcapable of more accurate and/or direct measurements of flow, without theneed for heating or cooling equipment, and to provide a straightforwardway to retrieve the measurements from the sensor. Such considerationscan apply to a wide range of applications involving the measurement ofgas and fluid flow, including CSF, within an implanted device or anembedded, encapsulated, or relatively inaccessible space.

Accordingly, there remains a need for non-invasive monitoring and/ormeasuring the flow of a fluid, and more particularly for monitoringand/or measuring the flow of a fluid in an implantable medical device.

SUMMARY

In one embodiment, an exemplary implantable sensor for measuring fluidflow is provided. The implantable sensor can have a sensor housingadapted to receive fluid flow therethrough. In some embodiments, thesensor housing can have a domed portion defining a reservoir therein,and the implantable sensor can measure the flow rate of fluid throughthe reservoir. A radio frequency tag can be located within the sensorhousing. The radio frequency tag can be adapted to interact with awireless signal and to produce a response to the wireless signal. Theimplantable sensor can also include a masking element that is disposedin the sensor housing. The masking element and the radio frequency tagcan be configured to move relative to one another, for example themasking element moving, the radio frequency tag moving, or both, toalter the response of the radio frequency tag and thereby indicate arate of fluid flowing through the sensor housing. The masking element,for example, can include a conductive member that alters the response ofthe radio frequency tag by covering at least a portion of it. Theresponse of the radio frequency tag can have at least onecharacteristic, such as a resonant frequency, harmonic spectra, decaycharacteristic, and Q factor, that corresponds to the flow rate. In someembodiments, the implantable sensor can also have a valve assembly thatis in fluid communication with the sensor housing and that is adapted tocontrol a rate of fluid flowing through the sensor housing.

A wide array of variations are possible. The radio frequency tag caninclude a disk having an asymmetrical antenna, and the masking elementcan be configured to mask at least part of the antenna. In someembodiments, the radio frequency tag can also include a chip for storingdata and an antenna adapted to communicate the stored data to anexternal reading device.

Various techniques can be used to associated the radio frequency tagwith the flow rate. For example, the masking element can be configuredto rotate in response to the flow rate of fluid through the sensorhousing. For example, the masking element can include a disk formed atleast in part of a conductive material and configured to rotate aroundan axis thereof. The masking element can be configured to rotate aroundan axis thereof to mask different parts of the radio frequency tag suchthat the response of the radio frequency tag to the wireless signal isperiodic. The rotation can selectively mask part of the radio frequencytag such that the response of the radio frequency tag to the wirelesssignal is periodic. The conductive material can be in the form of, forexample, a spiral or a plurality of discrete conductive sections. Inother embodiments, the masking element can include a disk having apattern of conductive material formed thereon and a biasing element,such as a spring, calibrated to resist rotation of the disk caused bythe flow rate of fluid through the sensor housing. Alternatively, themasking element can have a wedge formed at least in part of a conductivematerial and the masking element can be configured to translate inrelation to the flow rate of fluid through the sensor housing.

In another embodiment, an implantable sensor is provided which has asensor housing adapted to receive fluid flow therethrough, and aconductive member disposed within the valve assembly. The conductivemember can be configured to selectively cover at least a portion of aradio frequency tag and thereby alter a response thereof to a wirelesssignal to indicate a rate of fluid flowing through the sensor housing.The response of the radio frequency tag can have at least one measurablecharacteristic, such as resonant frequency, harmonic spectra, decaycharacteristic, and Q factor, that can indicate the flow rate. In someembodiments, the frequency tag can be configured to move relative to theconductive member, which can result in the frequency tag beingselectively covered. The radio frequency tag can also include a diskhaving an asymmetrical antenna formed thereon. In other embodiments, theconductive member can be configured to move relative to the radiofrequency tag, which can result in the frequency tag being selectivelycovered. The conductive member can form part of a rotatable disk, forexample. In some embodiments, the implantable sensor can include a valveassembly that is in fluid communication with the sensor housing and thatis adapted to control the rate of fluid flowing through the sensorhousing.

In other aspects, an exemplary method for measuring fluid flow isprovided and can include positioning a sensor housing between an inlettube and an outlet tube, the sensor housing having a radio frequency tagdisposed therein. The method can further include transmitting a wirelesssignal to the radio frequency tag from a reading device, and wirelesslyreceiving a response from the radio frequency tag that indicates a rateof fluid flowing through the sensor housing. In some embodiments, theresponse can change in relation to the rate of fluid flowing through thesensor housing, and/or it can include a periodic signal. The method canalso include analyzing the response to detect any of resonant frequency,harmonic spectra, decay characteristic, and Q factor of an antennaincluded in the radio frequency tag, and/or receiving a response fromthe radio frequency tag that communicates data previously storedtherein.

In other embodiments, the method can further include coupling the inlettube to a catheter within a patient's ventricle, and coupling the outlettube to a drainage catheter for draining the patient's cerebrospinalfluid. The method can also include coupling the sensor housing to avalve assembly adapted to control a rate of fluid flowing through thesensor housing.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments disclosed herein will be more fullyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings, in which:

FIG. 1A is a top view of one exemplary implantable valve suitable foruse in a hydrocephalus shunt and including a flow sensor.

FIG. 1B is a schematic view of one exemplary embodiment of animplantable sensor having a housing with a radio frequency tag and amasking element disposed therein for measuring the flow of a fluidtherethrough;

FIG. 2A is a top view of one exemplary embodiment of a radio frequencytag and a masking element for use with the implantable sensor shown inFIG. 1B;

FIG. 2B is a top view of the radio frequency tag and masking elementshown in FIG. 2A following rotation of the masking element;

FIG. 3 is a schematic view of an exemplary configuration for couplingthe flow of a fluid to a radio frequency tag and/or masking element tocause rotation thereof;

FIG. 4 is a graph illustrating amplitude vs. frequency characteristicsof an exemplary response from a radio frequency tag in an implantablesensor;

FIG. 5 is a top view of another embodiment of a radio frequency tag anda masking element;

FIG. 6 is a top view of another embodiment of a radio frequency tag anda masking element;

FIG. 7A is a top view of another embodiment of a radio frequency tag anda masking element;

FIG. 7B is a top view the radio frequency tag and masking element shownin FIG. 7A following translation of the masking element and/or radiofrequency tag;

FIG. 8 is a schematic view of an exemplary configuration for couplingthe flow of a fluid to a radio frequency tag and/or masking element tocause translation thereof;

FIG. 9A is a schematic diagram of one exemplary model of a circuithaving resonance characteristics;

FIG. 9B is a graph of an output voltage signal as a function offrequency for the circuit shown in FIG. 9A;

FIG. 9C is a graph of an output voltage signal as a function offrequency for the circuit shown in FIG. 9A;

FIG. 10 is a perspective view of an exemplary reading device for readinga flow rate from an implantable sensor; and,

FIG. 11 illustrates the implantable sensor shown in FIG. 1 implanted ina body and being read by the reading device shown in FIG. 10.

DETAILED DESCRIPTION

Certain exemplary embodiments will now be described to provide anoverall understanding of the principles of the structure, function,manufacture, and use of the devices and methods disclosed herein. One ormore examples of these embodiments are illustrated in the accompanyingdrawings. Those skilled in the art will understand that the devices andmethods specifically described herein and illustrated in theaccompanying drawings are non-limiting exemplary embodiments and thatthe scope is defined solely by the claims. The features illustrated ordescribed in connection with one exemplary embodiment may be combinedwith the features of other embodiments. Such modifications andvariations are intended to be included within the scope of the presentapplication.

The present application generally provides methods and devices fornon-invasively measuring or quantifying the flow of a fluid through asdevice and for indicating that information to another device, e.g.,telemetrically. The methods and devices are particularly useful in thecontext of implantable devices, such as hydrocephalus shunts andassociated valves. While the description herein sometimes refers tohydrocephalus shunts, one skilled in the art will understand that thedevices and methods described herein can be used in a wide variety ofmethods and devices in which it is desirable to measure fluid flow.

FIG. 1A illustrates one exemplary embodiment of an implantable sensor inthe context of an implantable valve for a hydrocephalus shunt. As shown,the implantable valve 101 can include a housing 102 with proximal anddistal ends 104 a, 104 b for receiving fluid flow (such as CSF)therethrough between an inlet port 106 and an outlet port 110. Thehousing 102 can have virtually any configuration, shape, and size. Inmany embodiments, the size and shape of the housing 102 can be adaptedfor implantation in a body, e.g., subcutaneous implantation. In theembodiment shown in FIG. 1A, the housing 102 has a substantially linearconfiguration with a reservoir 108 having a larger area than the ports106, 110 which can be advantageous for checking the shunt's patency,tapping the CSF, or to administer therapy. The reservoir 108 can alsohouse a flow sensor, as will be described in more detailed below, andcan also house a pressure sensor for measuring the pressure of fluid inthe reservoir. For example, suitable pressure sensors are described inco-pending, commonly assigned U.S. patent application Ser. No.10/907,665, entitled “Pressure Sensing Valve” by Mauge et al., filedApr. 11, 2005 (now published as U.S. Publication No. 2006-0211946 A1),filed herewith, and in U.S. Pat. No. 5,321,989, U.S. Pat. No. 5,431,057,and EP Patent No. 1 312 302, the teachings of all of which are herebyincorporated by reference in their entireties. The implantable valve 101can also include a valve assembly 112 for controlling the flow of fluidthrough the valve 101 according to remotely or telemetrically selectablesettings. A coating can be disposed over the valve 101. Furtherinformation on implantable valves can be obtained from U.S. PublicationNo. 2006-0211946 A1, referenced above.

FIG. 1B schematically illustrates one exemplary embodiment of animplantable sensor in the housing 102. Fluid (e.g., CSF) can flow asindicated by directional arrows through the housing 102 from an inlet(fluid entry) port 106 at the proximal end 104 a, through a reservoir108, and out an outlet (fluid exit) port 110 at the distal end 104 b.The reservoir 108 is not necessary (for example, the housing can be inthe form of a catheter or tube), but can be advantageous foraccommodating other components of the sensor or device therein. Thesensor 100 can also have a radio frequency (RF) tag 114 disposed in thereservoir 108, a masking element 115 associated with the RF tag 114 (forclarity, the RF tag 114 and the masking element 115 are representedtogether in FIG. 1B, although they need not be one component, or besimilarly sized and oriented). Moreover, while the RF tag and maskingelement are shown disposed within the reservoir 108, they can bepositioned at any location within a fluid system as long as they can beused to indicate a flow rate of fluid in the system. As will describedin more detail below, the RF tag 114 and the masking element 115 can beconfigured to move relative to one another in response to and/or inrelation to the rate of flow of fluid through the housing, and toindicate the rate of fluid flow to an external reading device. In someembodiments, the RF tag 114 can further store data, such asidentification information (for example, for the sensor and/or for thepatient) and flow rate history, which can be communicated to theexternal reading device. While not shown, the housing 102 can alsocontain a valve assembly for controlling the flow of fluid from theinlet port 106 to the outlet port 110, the sensor 100 measuring thecontrolled flow of fluid therethrough. The proximal and distal ends 104a, 104 b of the sensor 100 can each be open and adapted to couple toanother medical device, such as a catheter.

The masking element can have a wide variety of configurations and it canbe adapted to interact with the RF tag in a variety of ways. FIG. 2Ashows one exemplary embodiment of a masking element 115 associated withan RF tag 114 in which the masking element 115 is in the form of a diskwith a conductive portion 202 and a non-conductive (or differentlyconductive) portion 208. The conductive portion 202 can be a material,such as silver, gold, copper, aluminum, or others known in the art,etc., deposited on the disk. The conductive potion 202 can also beattached or coupled to the disk, or it can be a non-circular portionthat fits together with a non-conductive portion 208 to form thecomplete disk, and so on. The conductive portion 202 can have a varietyof shapes, but as shown it is in the shape of a spiral. Alternatively,the conductive portion can be in the shape of a strip of varying width,can have one or more discrete portions of varying size (e.g., aplurality of discrete rectangular shapes), and can have virtually anysize and shape that is rotationally asymmetric. As shown in FIG. 2A, theRF tag 114 can be disposed behind the masking element, and particularlybehind the spiral portion formed of conductive material 202. (In otherembodiments, the RF tag can be disposed behind or in front of themasking element.) In use, the flow of a fluid through a housing 102,such as is shown in FIG. 1, can cause rotation of the masking element115 about the central axis 206, while the RF tag 114 can remain fixed(for example, fixed relative to the housing shown in FIGS. 1A-1B). FIG.2B illustrates one possible result of such relative movement. As shownin FIG. 2B, following rotation of the masking element 115, a narrowportion of the conductive material 202 covers the RF tag 114.Accordingly, the response of the RF tag 114 to an external signal (e.g.,from a reading device emitting a signal at one or more radiofrequencies) in FIG. 2A can differ from that of FIG. 2B and can indicatesuch relative position and/or movement. For example, in someembodiments, a characteristic of the response of the RF tag 114, such asresonance frequency, harmonic spectra, decay characteristics, or Qfactor, can change depending on the relative position or motion of themasking element 115 and the RF tag 114, to indicate the flow rate offluid within the sensor housing.

In some embodiments, the masking element 115 can rotate around itscentral axis 206 at a speed relative to the RF tag 114 to indicate afluid flow rate. For example, as shown in FIG. 3, an element 300 can becoupled to fluid flowing in direction 302 in a housing 102 via surfacefeatures 304 formed on the element 300, causing the element 300 torotate as shown by arrow 306. The element 300 can be a masking element115 itself or alternatively it can be coupled to the masking element115, for example, via a shaft, gears, and so on. The resulting relativemotion of the masking element 115 and the RF tag 114 can be manifestedas a periodic radio frequency signal, such as is shown in FIG. 4. Theperiod of the signal (for example, the periodicity of the f_(max) peaksin FIG. 4, or other metric) can be correlated to the flow rate of thefluid.

It should be understood that in some embodiments the nature of therotation of the masking element 115 relative to the RF tag 114 can varyand can be used to sense or measure other characteristics. In someembodiments, in response to a pressure from flowing fluid the maskingelement 115 can rotate to a position relative to the RF tag 114 andstop. For example, as shown in FIG. 3, a spring 308 can be disposed onthe rotational axis of element 300 to resist the rotation thereof, andthe spring 308 can be calibrated such that a given force or pressurecauses a known degree of rotation, or rotational deflection, in theelement 300. The resulting relative position of the masking element 115and the RF tag can indicate the pressure via the response of the RF tagto a radio frequency signal, as described above.

The masking element 115 and the RF tag 114 shown in FIG. 1B can have awide variety of other configurations. For example, FIG. 5 shows anexemplary masking element 500 in the form of a disk which includes aconductive portion 504 disposed within a disk of non-conductive, ordifferently conductive, material 508. As shown, the conductive portion504 is in the form of a circle sized to cover the RF tag 502 completely,although in other embodiments, the conductive portion can be adapted tocover the RF tag 502 partially. The masking element 500 can be coupledto flowing fluid so as to rotate around a central axis 506 in relationto the rate of flow, as previously described. Rotation of the maskingelement 500 can result in a periodic sudden change or discontinuity inthe RF tag's response and thereby indicate the flow rate.

In another embodiment, shown in FIG. 6, a masking element 600 can be inthe form of a rectangle, square, or virtually any other shape, and itcan be associated with an RF tag 602 having an asymmetric shape. Forexample, the RF tag 602 can be in the form of a disk with a rotationallyasymmetric antenna pattern formed thereon. The pattern can include, forexample, antenna lines with varying width, spacing, orientation, and soon. The masking element 600 can be fixed within the housing, and thedisk forming the RF tag 602 can be coupled to flowing fluid, e.g., inthe housing as shown in FIG. 1B, so as to rotate around axis 604 inrelation to the flow rate, as previously described. Such rotation cancause a change or variations in the response of the RF tag 602 as theconductive masking element 600 covers different portions of theasymmetric antenna of the RF tag 602. As previously mentioned, theresponse can have characteristics, such as resonance frequency, harmonicspectra, decay characteristics, and/or Q factor, which can change as aresult of such rotation and which can be detected in the response of theRF tag 602 to a radio frequency signal emitted by a reading device. Inan alternative embodiment, the RF tag 602 can be fixed within thehousing and the masking element 600 can be adapted to rotate around anaxis or otherwise move relative to the RF tag 602.

In yet another embodiment, the masking element can be configured totranslate relative to the RF tag. For example, FIG. 7A shows a maskingelement 700 formed of a conductive material in the shape of a wedge andassociated with an RF tag 702. As the masking element 700 translatesrelative to the RF tag 702, it covers a different portion of the RF tag702 (for example as shown in FIG. 7B), resulting a measurable differencein the RF tag's response, as previously described. The masking element700 can be coupled to the flow of fluid through the sensor housing in avariety of configurations. For example, the configuration describedabove in connection with FIG. 3 can be adapted such that rotation of theelement 300 causes translation of the masking element 700, for examplevia a rack and pinion gearing.

Alternatively, as shown in FIG. 8, a sliding element 800 can be disposedin a housing 102. The sliding element 800 can be configured to receive aforce of fluid flowing in direction 802 against a proximal end 804thereof or against elements 806, and to translate in response thereto. Aspring 808 or other biasing element can be configured to provide a forceagainst a distal end 810 of the sliding element 800 that resists theforce presented on the sliding element 800 by flowing fluid, and thespring 804 can be calibrated such that the deflection of the slidingelement 800 corresponds to a force or pressure from the fluid flow. Thesliding element 800 can be coupled to a masking element, such as maskingelement 700, to effect translational movement thereof.

As one skilled in the art will appreciate, the masking element and theRF tag can have a wide variety of further configurations, includingvirtually any configuration in which a masking element and an RF tagmove relative to one another to measure a rate of fluid flow and/orpressure. For example, in some embodiments a variety of masking elementshapes can be provided, in some embodiments only one or both of themasking element and the RF tag can be configured to move relative to theother, and so on. In some embodiments, the masking element canphysically contact the circuit of the RF tag to thereby change itsproperties (resistance, capacitance, inductance, etc.) and/or alterconnections between conductive elements on the RF tag, for exampleconnecting conductive branches of a circuit, or breaking suchconnections. In other embodiments, the masking element covers or isdisposed in between the reading device and the RF tag. The location ofthe RF tag and masking element can vary within the housing and are notlimited to those shown in the illustrated embodiments. In addition, anymechanism suitable to convert the flow of a fluid to rotational ortranslational movement can be provided, the foregoing embodiments beingby way of example only. Further, many of the embodiments describedherein can be adapted to determine or can be correlated to a pressure offluid in a housing rather than a flow rate.

Returning to FIGS. 1A-1B, the shape, technical specifications, and sizeof the RF tag 114 can vary widely. In many embodiments, a relativelysmall RF tag can be used so as to minimize the footprint of the tag inthe device, for example with dimensions in a range of about 5 mm to 10mm, but in other embodiments, tags with dimensions of about 3 mm to 50mm can be used and any size is possible.

It should be understood that in many embodiments, the RF tag 114 can bechipless, and its physical/electromagnetic parameters can be used todetermine a flow rate. The RF tag 114 need not have the capability tostore data or to communicate according to a protocol, and need not haveprocessing circuitry or digital logic. A chipless RF tag can provide acircuit (for example, having measurable characteristics, such as a tankcircuit) and can be powered from the reading device signal. Such an RFtag can be advantageous due to its relatively low power requirements,and need not have the ability to communicate stored data or “identify”itself. However, in other embodiments the RF tag 114 can be chip-based,and can provide data storage for storing additional information relatedto the application. An example of chip-based tags are the commonly usedRF identification tags. Some of these RF identification tags provideminimal information (such as a TRUE or FALSE value), while others canstore several bytes of data. A chip-based RF tag can include processingcircuitry, digital logic, a separate antenna, and/or a battery. Forexample, the RF tag 114 can include a memory for storing data related tothe patient and/or sensor. By way of non-limiting example, the RF tag114 can store sensed pressure data, sensor identification information(e.g., implantation date, sensor type, and sensor identifier code),sensor calibration data, historical data stored from the sensor, tagidentification information (e.g., implantation date, tag type, and tagidentifier code), and/or patient data (e.g., desired CSF flow rate,previous sensor measurements, and patient medical history). An externalreading device, described further below, can read and/or store data insuch an RF tag 114.

The RF tag 114 can have any shape, such as elliptical (includingcircular) or rectangular (including square), and can have virtually anysize. The following table lists, by way of example only, available RFtags suitable for use with the devices and methods described herein.Passive as well as semi-passive and active tags can be used, althoughsemi-passive and active tags sometimes are larger than passive tagsbecause they may have an internal battery, e.g., for power purposes.

TABLE 1 Tag Frequency Type 125 KHz 5-7 MHz 13.56 MHz 303/433 MHz 860-960MHz 2.45 GHz Passive ISO11784/5, ISO10536 (ISO15693) — ISO18000-6ISO18000-4 14223 iPico (ISO15693) Electronic Product IntellitagISO18000-2 DF/iPX MIFARE Code (“EPC”) μ-chip (ISO14443) Class 0 Tag-ITEPC Class 1 (ISO15693) EPC GEN II ISO18000-3 Intellitag tolls (Title 21)rail (Association of American Railroads (“AAR”) S918) Semi- — — — — rail(AAR S918) ISO18000-4 Passive Title 21 Alien BAP Active — — — Savi(American — ISO18000-4 National Standards WhereNet Institute (“ANSI”)(ANSI 371.1) 371.2) ISO18000-7 RFCode

By way of further explanation, one exemplary circuit for modeling an RFtag can be generally represented by a resonator circuit 900 as shown inFIG. 9A. The circuit 900 includes a capacitor 902, an inductor 904, andan intrinsic resistance 906. When the RF tag is embedded in a sensor andassociated with a masking element, as described above, shifts in theresonant frequency of the circuit 900 can be monitored on a continuousor intermittent basis to measure a rate of fluid flow through thehousing. The resonant frequency of the circuit 900 can be detected in avariety of ways, such as by measuring power reflected from the circuit900 or measuring decaying circulating power of the circuit 900 followinga outside signal (e.g., from a reading device). FIG. 9B illustrates anexample of a graph showing an output signal of the circuit 900 whenintroduced to an outside signal. The reflected power of the circuit 900is at a minimum at the resonant frequency, where w can be expressed as:

$\omega = {{2\pi \; f} = \frac{1}{\sqrt{LC}}}$

with f representing the resonant frequency, L representing inductance ofthe inductor 904, and C representing capacitance of the capacitor 902.FIG. 9C illustrates another example of a graph showing an output signalof the circuit 900 when introduced to an outside signal. The reflectedpower of the circuit 900 in this example is at a maximum at the resonantfrequency. Further examples of such RF tags and information on the useof them, including techniques for interrogating them, can be obtainedfrom U.S. Pat. Nos. 6,025,725, and 6,278,379, and U.S. PatentApplication Publication No. 2004/0134991, all of which are hereby byincorporated by reference in their entireties.

Referring again to FIGS. 1A-1B, the housing 102 can be formed from avariety of materials. In one exemplary embodiment, however, the housing102 is formed from a flexible, biocompatible material. Suitablematerials include, for example, polymers such as silicones,polyethylene, and polyurethanes, all of which are known in the art. Thehousing 102 can also optionally be formed from a radio-opaque material.A person skilled in the art will appreciate that the materials are notlimited to those listed herein and that a variety of other biocompatiblematerials having the appropriate physical properties to enable thedesired performance characteristics can be used.

The valve 101, the sensor 100 and/or the RF tag 114 and masking element115 can also optionally include a coating 116 that is adapted tohermetically seal all or at least a portion of the RF tag 114 and/ormasking element 115. The coating 116 can be applied to only a portion ofthe RF tag 114 and/or masking element 115 that could be exposed tofluid. The RF tag 114 and the sensor 100 can be coated separately, withdifferent coatings, or together in a single coating. An adhesive orother mating technique can optionally be used to affix the RF tag 114and/or masking element 115 within the reservoir 108, however, in someembodiments it can be useful to allow the RF tag 114 and/or maskingelement 115 to be removed from the sensor 100 if necessary.Alternatively, the sensor 100 can be coated after the RF tag 114 and/ormasking element 115 are disposed in the reservoir 108 to form aprotective sheath. The ports 106, 110 can be protected from any coatingapplied thereto, formed after the coating is applied, or be cleared ofany coating applied thereto to allow fluid to flow therethrough. Inother embodiments, only certain components of the sensor 100 can becoated. A person skilled in the art will appreciate that a variety ofother techniques can be used to seal the components of the sensor 100.

The material used to form the coating 116 can vary, and a variety oftechniques can be used to apply the coating. By way of non-limitingexample, suitable materials include polyurethane, silicone,solvent-based polymer solutions, and any other polymer that will adhereto the components to which it is applied to, and suitable techniques forapplying the coating include spray-coating or dip-coating.

FIG. 10 shows one exemplary embodiment of a reading device 1000, such asan RF telemetry device, for use in obtaining information from the RF tag114. The reading device 1000 can emit a signal at one frequency or overa range of frequencies, and can listen for the response thereto, e.g.,from the RF tag 114. In the case of a chipless RF tag, a characteristicof the response from the tag can indicate a measured flow rate, asexplained previously. In the case of a chip-based RF tag having memoryassociated therewith, the response of the tag can communicateinformation (e.g., according to a communication protocol) stored in itsmemory for the reading device. Any type of external reading device canbe used. In one exemplary embodiment, the reading device 1000 caninclude an RF module (e.g., transmitter and receiver), a control unit(e.g., microcontroller), a coupling element to the transponder (e.g.,antenna), and an interface (e.g., Recommended Standard (RS) 232, RS-485,Firewire, Universal Serial Bus (USB), Bluetooth, ZigBee, etc.) to enablecommunication with another device (e.g., a personal computer). Thereading device 1000 can provide the power required by the RF tag 114 tooperate, e.g., via inductive coupling. As shown in FIG. 11, the readingdevice 1000 can be positioned in proximity to an implanted RF tag 114 totelemetrically communicate with the RF tag 114, and thereby obtain areading of the measured flow rate.

In another aspect, a method for measuring a rate of fluid flow isprovided. In one embodiment, an exemplary method can include implantinga flow sensor, such as the flow sensor 100 described above in connectionwith FIGS. 1A and 1B, in a body. In the case of a hydrocephalus shunt, ahydrocephalus valve including the flow sensor can be subcutaneouslyimplanted in a patient, as shown in FIG. 11. It should be understoodthat while FIG. 12 shows the implantation of a flow sensor in a shoulderregion, the device can be implanted virtually anywhere, for examplesubcutaneously behind the ear, or on the head, torso, etc. The methodcan also include coupling a proximal end of a catheter, such as aventricular catheter, to an inlet port of the flow sensor. Anothercatheter, such as a drainage catheter, can be coupled to an outlet portof the flow sensor. The drainage catheter can extend through the patientto an area where excess fluid, e.g., CSF, can drain safely.

The method can further include wirelessly transmitting a wireless signalto an RF tag embedded in the flow sensor, for example using a readingdevice such as reading device 1000 described above in connection withFIG. 10. The transmitted signal can be include one or more frequencies.In some embodiments, the wireless signal can be transmitted according toa protocol in order to communicate with an RF tag having a chip therein.The method can also include receiving a response from the RF tag thatindicates a rate of fluid flowing through the sensor housing. Theresponse can have one or more characteristics, such as resonancefrequency, harmonic spectra, decay characteristics, and Q factor, thatcan be detected and analyzed in order to determine a measured rate offlow. In some embodiments, the response from the RF tag can be static,not changing over time unless the rate of fluid flow changes. In otherembodiments, the response from the RF tag can exhibit periodicity, andanalysis of the response can include determining a rate of flow based onthe periodicity of the response signal. The determination of a flow ratecan be performed using calibration data for a particular flow sensor. Insome embodiments, the calibration data, as well as other data such asidentification and/or historical data, can be transmitted from an RF taghaving a memory to the reading device.

Further information on wireless shunts can be obtained from U.S. Pat.No. 7,842,004 entitled “Wireless Pressure Setting Indicator” by SalimKassem, U.S. patent application Ser. No. 11/931,151 entitled “WirelessPressure Sensing Shunts” by Salim Kassem, and U.S. patent applicationSer. No. 11/931,187 entitled “Wireless Shunts With Storage” by SalimKassem, all of which were filed on Oct. 31, 2007 and which are herebyincorporated by reference in their entirety. Also incorporated byreference in its entirety is commonly assigned U.S. Pat. No. 7,510,533,entitled “Pressure Sensing Valve.”

A person skilled in the art will appreciate that the various methods anddevices disclosed herein can be formed from a variety of materials.Moreover, particular components can be implantable and in suchembodiments the components can be formed from various biocompatiblematerials known in the art. Exemplary biocompatible materials include,by way of non-limiting example, composite plastic materials,biocompatible metals and alloys such as stainless steel, titanium,titanium alloys and cobalt-chromium alloys, glass, and any othermaterial that is biologically compatible and non-toxic to the humanbody.

One skilled in the art will appreciate further features and advantagesbased on the above-described embodiments. Accordingly, the disclosure isnot to be limited by what has been particularly shown and described,except as indicated by the appended claims. All publications andreferences cited herein are expressly incorporated herein by referencein their entirety.

What is claimed is:
 1. An implantable sensor, comprising: a sensorhousing adapted to receive fluid flow therethrough; and a conductivemember disposed within the valve assembly and configured to selectivelycover at least a portion of a radio frequency tag and thereby alter aresponse thereof to a received wireless signal to indicate a rate offluid flowing through the sensor housing.
 2. The implantable sensor ofclaim 1, wherein the response has at least one measurable characteristicselected from the group consisting of resonant frequency, harmonicspectra, decay characteristic, and Q factor.
 3. The implantable sensorof claim 1, wherein the conductive member is configured to move relativeto the radio frequency tag.
 4. The implantable sensor of claim 1,wherein the radio frequency tag is configured to move relative to theconductive member.
 5. The implantable sensor of claim 1, wherein theradio frequency tag comprises a disk having an asymmetrical antennaformed thereon.
 6. The implantable sensor of claim 1, further comprisinga valve assembly adapted to control a rate of fluid flowing through thesensor housing, the valve assembly being in fluid communication with thesensor housing.
 7. The implantable sensor of claim 1, wherein theconductive member forms part of a rotatable disk.